1. Field of Invention
This invention relates to optics. Specifically, the present invention relates to optical devices used for separating white light into red, green, and blue light, and devices used for combining red, green, and blue light into white light.
2. Description of the Related Art
The principle of tristimulus colorimetry is used in optical imaging or display systems to generate the full range of colors. Most display systems utilize the red green blue tristimulus system in which any color is created from the appropriate combination of red, green, and blue. White light occupies part of the electromagnetic spectrum ranging from approximately 4.3.times.1014 Hertz to 7.5.times.1014 Hertz and within this spectrum blue and red light occupy the high and low frequency ends respectively while green represents the central frequency band of the visible spectrum.
Specific examples of display systems where the invention is applicable are Liquid Crystal Displays (LCDs) and reflective Digital Micromirror Devices (DMDs). These systems generally employ three such display panels one each for color red, green, and blue.
Another application is in the area of head-up display systems in which information is projected on the windscreen of a automobile, plane or helicopter. These applications require compact optical components to provide separate beams of red, green, and blue light using either polarized or unpolarized white light as a source.
Certain display and imaging systems utilize the principle of thin film interference to separate frequency bands of white light to generate red, green, and blue. Specifically, color separation is often achieved using frequency sensitive optical multi-layer coatings of dielectric materials applied to glass substrates. The tristimulus form of color separation of white light into red, green, and blue is accomplished when these coatings are applied to a number of color selective, i.e. dichroic, mirrors. The dichroic mirrors are then arranged in any number of configurations to create channels of red, green, and blue light. However, these arrangements are bulky and require expensive support structures to support the mirrors and lenses in the desired configuration. In addition, these arrangements require lenses with long back focal lengths when used in imaging systems.
To reduce bulkiness and the need for lenses with long back focal lengths, the dichroic coatings can be mounted inside a cubic glass beamsplitter at a 45 degree angle of incidence. Although the cubic beamsplitter is compact, the coatings cemented in glass at a 45 degree angle of incidence exhibit certain limitations on performance. The performance problems are manifested as a sensitivity to the polarization and the angle of incidence of the incoming beam.
Nominally, light enters the cubic beamsplitter at a forty-five degree angle of incidence to the dichroic surfaces. This large angle introduces undesirable polarization effects resulting in an unclear splitting of the red, blue, and green light. To minimize these undesirable effects, polarized light is used with the cubic beamsplitters. The use of polarized light forces a compromise in image intensity which may be unacceptable for some applications.
Hence, a need exists in the art for a cost effective, compact system that can cleanly split white light in an orthogonal configuration, with minimal polarization and angle sensitivity, into red, green, and blue light.